Recently, the widespread use of Internet allows users to access and obtain various information provided at sites that are run in various places of the world. This leads to the widespread use of broadband accesses such as ones through ADSLs (Asymmetric Digital Subscriber Lines) and FTTH (Fiber To The Home) including a PON, too.
An optical fiber link is a technique that is most likely to satisfy the service providers' demand that a high bandwidth communication be combined with long reach of economic development. An optical fiber, which has taken over a long-haul network backbone, is emerging in metropolitan areas. Access networks, however, have a large number of terminal points, and therefore require a different network topology than SONET (Synchronous Optical Network) or DWDM (Dense Wavelength Division Multiplexing) which are used in long-haul networks and metro networks.
A passive optical network (PON), which includes an optical line terminal (OLT) in a telephone station, optical network units (ONUs) at subscribers' residences, shared fiber networks, and passive optical splitters, offers powerful advantages for broadband access networks. For several decades, Ethernet (a registered trademark) (IEEE 802.3) has been widely used in local area networks. More recently, Ethernet (a registered trademark) is a general-purpose LAN that has been used with increasing frequency in metro and other wide area optical network applications, including passive optical networks. In the following, for convenience of the description, Ethernet (a registered trademark) is simply referred to as “general-purpose LAN”.
Passive optical networks (EPONs), in gigabit general-purpose LANs and general-purpose LANs based on the newly emerging IEEE 802.3std (Section5) protocol standard, offer a high capacity and a low cost necessary for wide economic development.
As specified in IEEE 802.3ah, passive optical networks in general-purpose LANs have a communication range of up to 20 km, and an area supported by one PON interface in one OLT ranges widely. The number of connectable ONUs is up to 64. In a case where a failure occurs not in the ONU but in a transmission channel, that is, in a case where disconnection of an optical fiber or a failure of an optical splitter occurs, it is very difficult to identify the location at which the failure is occurring by human labor.
Patent Document 1, which shows a PON system, discloses a method for enhancing the accuracy of estimate of a location where a malfunction is occurring in a passive optical network based on a general-purpose LAN including optical network units. In this method, a threshold value determination part of a station-side device that receives PON topology information (information of the distance of the optical transmission channel and the stage order of the optical splitter) sets a threshold value for the number of times an error occurs in general-purpose LAN frames that are received per unit time with respect to each residence-side device. The threshold value is set to be a value resulting from adding, to a predetermined value, a value obtained by multiplying the distance of the optical transmission channel by a factor or a value obtained by multiplying the stage order of the optical splitter by a factor. As for the optical splitter, too, a threshold value is set by multiplying, by a factor, the sum of threshold values of all the residence-side devices that belong to the optical splitter. Upon reception of an error detection signal from an error detection part, the determination part obtains the number of times an error occurred per unit time in the residence-side device or in the optical splitter, and compares the obtained value with the threshold value, to determine whether or not a malfunction occurred.